A brief history of silicon
Siliceous sands and clays have been used as building and craft materials
since the earliest of times. However, since silicon does not occur
naturally in its free form, it was not isolated as the element until
the last century.
The first person to accomplish this was J.J. Berzelius, who reduced
silicon tetra fluoride with potassium metal in 1823. In 1854, H.E.
Sainte-Claire Deville succeeded in obtaining pure silicon by means
of molten salt electrolysis.
Half a century later, in 1904, F.S. Kipping unlocked the door to a
whole new chemistry of products when he prepared the first organochlorosilanes.
In 1940/41, Professors Müller and Rochow independently presented
a direct method for synthesizing polysiloxanes (silicones) on an industrial
scale.
From natural element to chemical building block
Survey of silanes and silicones
Silanes
Silanes are the raw materials for manufacturing silicones. They are
produced by direct reaction between silicon and methyl chloride in
the Müller- Rochow synthesis. They are extremely mobile, clear,
colorless liquids that are soluble in organic solvents and, in some
cases, in anhydrous alcohol. Silanes have low molecular weights
and are thus highly volatile.
Silicones
Unusual properties
Silicones, known chemically as polyorganosiloxanes, have a structure
similar to that of quartz , with a backbone consisting of alternating
silicon and oxygen atoms.
Their properties can be modified by attaching different organic groups
to the backbone. This is why silicones come in various forms, ranging
from resins right through to fluids.
A typical linear silicone polymer is shown on the cover. The methyl
groups are able to rotate freely about the silicon-oxygen backbone.
Half metal, half plastic
Silicones are a special class of plastic. Whereas plastics are generally
considered to be organic (carbon based) compounds, the silicon atom
also possesses the properties of a semi-metal, even though it is very
similar to the carbon atom. Silicones can thus be termed semi organic
compounds.
Remarkable stability
The linkages formed between silicon and oxygen are extremely strong.
As a result, silicones are highly resistant to very high and low temperatures,
UV and IR radiation, and countless other influences. Silicone fluids
have a low surface tension and hence great wetting power - they are
outstanding water repellents.
Chemical structure:
The key to product diversity
Silicones have unique chemical structure that allows them to be produced
in a wide variety of forms. The term "silicone" was coined
from "silico" and "ketone" because of the similarity
between the molecular formulary of silicones and that of ketones.
Otherwise, ketones and silicones have very few properties in common.
The former are liquid or crystalline monomers whereas the latter
are oily, resinous or rubbery polymers that are generally obtained
by hydrolysis
(and condensation) of silane monomer in the Müller- Rochow
synthesis.
The properties of silicones are determined by the parent silane.
Monofunctional silanes yield low molecular silicones and difunctional
silanes yield
high molecular ones. The latter are mainly cyclic if the attached
organic groups are short chained, and linear if they are long chained.
When
trifunctional silanes condense, the presence of the trifunctional
groups results in three dimensionally cross linked silicones.
Manufacture
In 1940/41, Professors Müller and Rochow succeeded independently
of each other in making silicon react with gaseous methyl chloride
(CH3CI) to yield a mixture of liquid methyl chlorosilanes, and in so
doing established a method for synthesizing the raw materials for silicones.
The silanes are converted in complex processes into silicone fluids,
rubbers and resins. These three classes of product are themselves
raw materials for other silicone products, such as greases, release
agents, antifoams, paint additives, paper coating agents, as well
as for water repellents for the construction, textiles and leather
industries,
and hot and cold curing rubber.
Chemical structure and properties
Because of their excellent release action, only silicone products are
currently used to make high-quality release papers. The product
line includes both solvent-based and solvent less systems that have
been
specially developed for making silicone release papers and firms.
Chemically, the silicones involved are polydimethylsiloxanes whose
ends bear cross
linkable hydroxyl or vinyl groups.
They are cross linked to the elastomer by either condensation
or addition reactions with silicic acid esters or hydride-functional
polysiloxanes.
